Combustion engines on the basis of Otto engines are generally operated with fuel consisting of hydrocarbons and fossil fuels on the basis of refined oil. Ethanol that has been increasingly produced from re-growing natural resources (plants) or another alcohol s added to this fuel in different mixing ratios. In the US and in Europe often a mixture of 70-85% ethanol and 15-30% gasoline is used under the trade name E85. The combustion engines are construed in such a way that they can be operated with pure gasoline as well as with mixtures up to E85; this is called ‘flex-fuel-operation’. For an economical operation with a low pollutant emission at a simultaneously high engine performance and good starting behavior the operating parameters in the flex-fuel-operation have to be adapted to the present fuel mixture. A stoichiometric air/fuel ratio is for example present at 14.7 weight proportions of air per proportion of gasoline, but an air percentage of 9 weight proportions has to be adjusted when using pure ethanol. Slight and/or slow changes of the alcohol content can be detected and considered by the engine management of the combustion engine with the aid of a lambda probe and/or a knock sensor. But for example after refueling also quick changes with a significant deviation of the composition of the fuel mixture can occur. If the combustion engine was operated with 100% gasoline and refueled with E85 at an almost empty tank, this can cause starting problems and complications with the combustion, which can also increase the exhaust gas emission. According to the state of the art such fast changes of the composition of the fuel can be detected with the aid of an alcohol sensor. But this component increases the costs of the combustion engine.
For operating diesel engines biodiesel-components, known as fame-fuels, are increasingly added to the diesel fuel that is won from raw oil. Furthermore fuels of the 2nd generation, produced from the entire biomass (cellulose), are known. These fuels, for example under the trade name SunFuel, are shortly before a series maturity and can also be added to the diesel fuel. Therefore the composition of the ultimately present fuel and related to this the combustion features of the fuel mixture can alternate in wide ranges.
Known are furthermore motor regulations, which take place on the basis of a direct, cylinder individual combustion chamber signals. Preferably cylinder pressure sensors are here used as a sensor, which determined the pressure history during a combustion cycle in temporal high-resolution. Thereby the average induced pressure pmi can be provided for example as a measure for the mechanical work that has been performed by the combustion engine and the status of the combustion focus MFB50 (mass fraction burnt 50%) that has been determined from the pressure history as a reference variable of the regulation. Such cylinder-pressure-based motor regulation concepts regulate the main effects of a change of the composition of the present fuel mixture onto the combustion almost completely away.
A procedure for determining the composition of a fuel mixture from at least a first fuel and a second fuel for operating a combustion engine is known from U.S. patent application Ser. No. 12/126,075, whereby the first and the second fuel provide different rates of combustion and/or different energy contents and whereby the combustion engine provides at least one pressure sensor in at least one combustion chamber, with which a time- and/or angle-synchronic pressure history in the combustion chamber is determined. Thereby it is provided that the composition of the fuel mixture is determined from the time- and/or angle-synchronic pressure history of the gas pressure in the at least one combustion chamber during a combustion phase.
This procedure has the disadvantage that it is a pure detection function, thus that a separate evaluation electronic for determining the composition of the fuel mixture from the data of the pressure sensor besides the pressure sensor has to be provided.
It is the task of the invention to provide a procedure, which allows a reliable and inexpensive detection of the composition of a fuel mixture or the quality of a fuel within an existing regulation concept for combustion engines.